The invention relates to radiography and particularly to equalized radiography which improves diagnostic capabilities by locally varying the radiation incident on the object to expose denser parts to more radiation and thereby to image all parts well despite limitations in the dynamic range and other characteristics of the imaging medium. More specifically, the invention pertains to selective equalization radiography which improves image quality by equalizing different fields of the image differently. A special case is exposing the lung field uniformly while equalizing other fields.
In conventional radiography, image quality and diagnostic value can be compromised when the object x-ray transmission variations exceed the limits of the imaging medium. For example, a good x-ray image of the lung field typically leaves the mediastinum and retrocardiac areas underexposed; if those areas are exposed well, then the lung field is overexposed. Equalization radiography can overcome such problems and can improve image quality and diagnostic value by varying the local x-ray exposure to compensate for local variations in the object's x-ray transmission. Such radiography is discussed in the commonly assigned Wang European Patent Application No. 86308224.4 and Aitkenhead and Gershman U.S. patent application Ser. No. 07/525,498 filed on May 18, 1990, as well as in Plewes U.S. Pat. No. 4,773,087 and U.S. patent application Ser. No. 07/242,644 filed Sept. 13, 1988. See, also, U.S. Pat. Nos. 4,675,893, 4,715,056, 4,677,652, and 4,741,012. All of the documents cited in this paragraph are hereby incorporated by reference in this specification. Further background material concerning the subject can be found in the documents made of record in the patents and applications cited in this paragraph.
In an exemplary equalization system, an x-ray fan shaped beam scans the patient and a modulator locally controls the x-rays before they reach the patient in order to modulate the radiation differently as between different sectors of the fan shaped and as between different stages of the scanning movement. The degree and kind of local modulation are under the conrol of a feedback circuit which locally measures the x-rays in the fan shaped exiting the object. The goal of this local, time varying modulation is to equalize the image by reducing the difference in exposure as between different areas at the image plane. The modulator can use a row of modulator elements which are individually and selectively movable into the x-ray beam to vary it locally such as by varying the local attenuation, the local beam cross-section, and/or the local exposure time of the x-rays impinging on the object.
An earlier proposal is discussed in Plewes, D. B., Computer Assisted Exposure In Scanned Film Radiography, Proceedings International Workshop On Physics and Engineering In Medical Imaging, March, 1982, pages 79-86. This proposal states at page 82, in connection with FIGS. 5 and 8, that while equalization may be helpful for nodule detection it may be detrimental for looking at diseases manifested by diffuse opacifications and for such a case the system feedback could be adjusted to maintain low spatial frequency over the lung field alone. The proposal shows in FIG. 5 a relationship setting forth a "conventional image" for the lung field and shows the result of a related experiment in FIG. 8. Other parts of the citation might be relevant as well.
While equalized radiography can provide significant improvement in image quality and diagnostic value, it can also introduce some image artifacts, as noted for example in Plewes, D. B. and Vogelstein, E., Exposure Artifacts in Raster Scanned Equalization Radiography, Med. Phys. Vol. 11. pp. 158-165 (1984) and in Vlasbloem, et al., RADIOLOGY, Vol. 169, pages 29-34 (October 1988). See, also, Plewes, D. B. et al., Maximizing Film Contrast For Scanning Equalization Radiography, Medical Physics, Vol. 17, No. 3, May/June 1990, pages 357-361, as well as the documents cited therein.